1. Field of the Invention
This invention relates to an optical pickup for a magneto-optical recording medium, and particularly to an optical pickup for a magneto-optical recording medium having wobbling tracks.
2. Background of the Invention
Information is recorded on a magneto-optical recording medium such as, for example, a magneto-optical disc by the Curie point recording method wherein the recording layer of the magneto-optical recording medium is magnetized perpendicularly in corresponding to the information being recorded. In the Curie point recording method, information is recorded on a magneto-optical recording medium by applying to a magnetic field perpendicular to the magneto-optical recording medium while heating the magneto-optical recording medium to above the Curie temperature with a light beam.
Information recorded on a magneto-optical disc serving as a magneto-optical recording medium is reproduced in the way shown in FIG. 1.
That is, a linearly polarized light beam emitted by for example, a laser diode 200 is reflected by the reflecting surface of a polarization beam splitter (PBS) 201 and enters an objective lens 202. This light beam is then converged by an objective lens 202 onto a magneto-optical disc D and is reflected. At this time, due to the so-called magnetic Kerr effect, the plane of polarization of the light beam is rotated to the right or to the left (by an angle .theta..sub.k or -.theta..sub.k), according to the direction of the magnetization of the magneto-optical disc D.
The reflected light beam from the disc D, whose plane of polarization has been rotated through for example .theta..sub.k by this magnetic Kerr effect, passes through the objective lens 202, enters and is transmitted through the PBS 201. At this time the rotation angle of the plane of polarization of the reflected light beam from the disc D is amplified by the polarization characteristics of the PBS 201.
That is, considering two orthogonal polarization components (which here will be called the x, y polarization components), and supposing that the PBS 201 has polarization characteristics such that the transmittance Tx of the x polarization component is 1 (=100%) and the transmittance Ty of the y polarization component is a predetermined value less than 1, as shown in FIG. 1, the light beam being transmitted through the PBS 201 causes the y polarization component only, of the polarization components, to be multiplied by Ty (&lt;1.0) and become smaller. As a result, as shown in FIG. 1, the rotation angle of the plane of polarization is amplified from .theta..sub.k to the greater .theta.'.sub.k.
The reflected light beam from the disc D, having been transmitted through the PBS 201, enters an analyzer 203 comprising for example a Wollaston prism. In the analyzer 203, an i polarization component and a j polarization component are detected from the reflected light beam from the disc D. The i and j polarization components respectively are the reflected light from the disc D projected onto the i and j axes of an i-j coordinate system obtained by rotating the x-y orthogonal coordinate system expressing the x, y polarization components counterclockwise through 45.degree..
Here, the x and y polarization components in this specification respectively correspond to the so-called S and P polarization components.
The i and j polarization components from the analyzer 203 are directed into detectors 204a, and 204b. The detectors 204a and 204b receive the i and j polarization components from the analyzer 203 with two respective light receiving surfaces and output voltages corresponding to the amounts of light they receive as signals I, J.
Because, as discussed above, the plane of polarization of the reflected light beam from the disc D has been rotated to the right or to the left (by rotation angle .theta..sub.k or -.theta..sub.k) according to the direction of the magnetization of the disc D, the magnitude relationship between signal I and signal J differs depending on this rotation direction. Consequently from this magnitude relationship it is possible to judge the magnetization direction of the disc D; that is, it is possible to reproduce the information recorded on the disc.
Accordingly, from signal I and signal J outputted from the detectors 204a and 204b, the difference ( I .sup.2 - J .sup.2) between the squares of their absolute values is computed as an MO signal and based on the size of this MO signal the information recorded on the disc D is reproduced.
A magneto-optical disc reproduction device which reproduces information in the way described above is shown in FIG. 2. A laser diode 1 emits a divergent light beam having a polarization direction of, for example the horizontal direction in FIG. 2. This light beam enters a collimator lens 4 which changes it into parallel light and discharges it to a grating 2. Based on this parallel light beam the grating 2 produces a main or primary beam for reproducing information recorded on a magneto-optical disc 103 and two auxiliary or secondary beams for detecting tracking error which are each a predetermined distance, for example 1/4 of the track pitch of the disc 103, from the main beam in a direction orthogonal to the track and are point-symmetrical about the main beam, and directs them toward a polarization beam splitter (PBS) 101.
The one main beam and two auxiliary beams from the grating 2 are transmitted through the reflecting surface 101a of the PBS 101, enter the objective lens 5, are converged onto the disc 103 and are reflected. At this time, as explained above, the plane of polarization of the light beams is rotated by the magnetic Kerr effect.
The light beam reflected by the disc 103, i.e. the reflected light from the disc 103, passes through the objective lens 5 and enters the PBS 101, is reflected through 90.degree. by the reflecting surface 101a in the PBS 101 and is thereby split from the parallel light path 111. The reflected light split from the parallel light path 111 is again reflected through 90.degree. by a reflecting surface 101b in the PBS 101 and directed into a Wollaston prism 7. Based on the light beam entering it, the Wollaston prism 7 produces three light beams: the original beam (hereinafter referred to as the i+j polarization component), an i polarization component, and a j polarization component, and discharges them toward a cylindrical lens 102.
The light beams discharged from the Wollaston prism 7 consist of parallel light, but this parallel light is converted by the cylindrical lens 102 into converging light. The converging light from the cylindrical lens 102 produces an astigmatism and is converged through a converging multilens 8 into a detector 9 of the kind shown in FIG. 3 having light receiving surfaces A through D, E, F, I and J.
In this case, where the reflected light corresponding to the main beam produced by the Wollaston prism 7, the i+j polarization component, the i polarization component, and the j polarization component are respectively converged onto the light receiving surfaces A through D of the central portion and the light receiving surfaces I and J shown in FIG. 3. 0f the reflected light corresponding to one of the auxiliary beams produced by the Wollaston prism 7, the (i+j) polarization component, the i polarization component and the j polarization component are respectively converged onto the light receiving surface E and the portions to the left and right thereof where there are no light receiving surfaces. 0f the reflected light corresponding to the other auxiliary beam, the (i+j) polarization component, the i polarization component and the j polarization component are respectively converged onto the light receiving surface F and the portions to the left and right thereof where there are no light receiving surfaces.
Voltages A through D, E, F, I and J corresponding to the amount of light received are outputted from the light receiving surfaces A through D, E, F, I respectively in the detector 9. In a processing circuit not shown in the drawings, ( I .sup.2 - J .sup.2) is computed and an MO signal is obtained. Also, {(A+C)-(B+D)} and (E-F) are computed and a focus error signal and a tracking signal are generated. A focus servo and a tracking servo are controlled to bring the focus error signal and the tracking error signal to 0. (I+J) is computed and output level control of the laser diode 1, etc, is carried out based on a pit signal obtained as a result of this computation.
In a magneto-optical disc reproduction device of the kind described above, because the beam splitter 101 is disposed in the light path of the parallel light 111 between the collimator lens 4 and the objective lens 5, the cylindrical lens 102 is necessary to make the parallel reflected light from the disc 103 into converging light. Consequently there has been the problem that the number of parts constituting the device is great and the light path to the detector 9 of the light beam emitted from the laser diode 1 is long and the device is large and expensive.
There have been devices (hereinafter referred to as divergent/convergent light optical system devices) wherein to avoid this problem the collimator lens 4 disposed between the Laser diode 1 and the grating 2 is disposed between the PBS 101 and the objective lens 5; in other words, the PBS 101 is disposed not in the parallel light path between the collimator lens 4 and the objective lens 5 but rather in the divergent/convergent light path (a divergent light path when seen from the laser diode 1 side and a convergent light path when seen from the disc 103 side) between the laser diode 1 and the collimator lens 4.
In this case, because the reflected light from the disc 103 which enters the PBS 101 is converging light, the cylindrical lens 102 can be dispensed with.
As the magneto-optical disc 103, there are those on which pregrooves are formed so that the tracks become wobbling tracks. Wobbling tracks are wavelike tracks having a predetermined frequency such as for example 22.05 kHz, formed a minute distance to the left and right of the center of the track proper; addresses on the disc 103 can be determined based on the phase of these wobbling tracks, and therefore there is no need to record addresses on the disc 103 and more information can be recorded.
When information recorded on such a magneto-optical disc 103 having wobbling tracks is to be reproduced by the divergent/convergent light optical system device discussed above, laser light shone onto the disc 103 is diffracted by the adjacent pregrooves and beams of diffracted light of the kind shown in FIG. 4 (the circular portions shown with broken lines in the drawing) are produced.
This diffracted light interferes with the reflected light from the track (the circular portion shown with solid lines in the drawing), and consequently a beam spot having areas of differing strengths is formed on the light receiving surfaces of the detector 9, as shown in FIG. 5.
This results in a tracking error signal fluctuating at the high frequency of 22.05 kHz being supplied to the servo circuit; however, the pickup servo system cannot follow up a tracking error signal of this high frequency, and as a result just follows up the average value of the tracking error signal fluctuating at 22.05 kHz. Consequently, even though the disc has wobbling tracks formed on it, tracking control is carried out so that the light beam is shone onto the center of the track proper (the center of the circular track which is not a wobbling track).
As a result, the position of the beam spot of the light beam on the disc 103 with wobbling tracks formed on it changes at 22.05 kHz, as shown in FIG. 6, as follows: ##STR1##
Accordingly, the influence of the diffraction caused by the pregrooves adjacent to the track also changes, and consequently a beam spot is formed on the light receiving surfaces of the detector 9, as shown in FIG. 7 whose strength distribution in the portions intersecting orthogonally with the track changes at a frequency of 22.05 kHz.
Here, when information is to be reproduced from the magneto-optical disc 103, because this is done based on the rotation direction of the plane of polarization, changes in the light strength of the beam spot formed on the light receiving surfaces of the detector 9 alone do not have any great affect on the reproduction of information from the magneto-optical disc 103.
On the other hand, the polarization characteristics of not just the PBS 101 but all PBSs are angle-dependent. That is, the reflected light reflected by the reflecting surface 101a of the PBS 101 and the transmitted light transmitted by the reflecting surface 101a are rotated according to the angle at which the light beam is incident on the reflecting surface 101a. Therefore, when the light beam is not parallel but rather is divergent or convergent, the plane of polarization of the beam spot formed on the detector 9 does not have the same direction over the whole beam spot.
Here, the fact that due to the angle-dependence of the PBS 101 the direction of the plane of polarization is not the same over the whole beam spot results in an error being included in the voltage outputted based on the direction of the plane of polarization. However, as long as the light strength and the direction of the plane of polarization of the beam spot do not change, this error voltage is of a constant value and can be measured in advance. Therefore, just the direction of the plane of polarization in the beam spot not being the same over the whole beam spot does not have any great affect on the reproduction of information from the magneto-optical disc 103.
However, when the magneto-optical disc 103 is a magneto-optical disc having wobbling tracks and is to be reproduced by a divergent/convergent light optical system device, a light beam of which the direction of the plane of polarization is not uniform and of which the light strength distribution changes at a frequency of 22.05 kHz due to interference enters the Wollaston prism 7.
Consequently, in this case, there is the problem that the voltage outputted from the detector 9 includes an error corresponding to the degree of the interference, in other words an error which changes with changes in the light strength of the beam spot, and it is not possible to obtain an accurate MO signal. Because of this it has been thought difficult to reproduce a magneto-optical disc 103 of which the tracks are wobbling tracks with a divergent/convergent light optical system device.
One conceivable method of solving this problem is to remove the wobbling track 22.05 kHz frequency component with a BPF (Band Pass Filter). However, signal components recorded on the magneto-optical disc 103 are included in the 22.05 kHz frequency band and it has been difficult to remove the wobbling track 22.05 kHz frequency component only.